Various movement or position encoders for sensing 1-dimensional (1D) linear, rotary or angular movement are currently available. These encoders are generally based on either optical systems, magnetic scales, inductive transducers, or capacitive transducers. Certain of these encoders are designed for making relative measurements. In such relative displacement or position encoders, measurements are typically made by sensing the relative change in position of the scales with respect to a reference position, which requires a sensing of the change in the scale pattern so that repetitions of the pattern can be counted. This type of position measurement may be referred to as incremental displacement measurement or incremental position sensing or measurement.
For optical encoders, a number of 1D incremental position systems have been developed. One recent system utilizing fewer parts than most previous systems is disclosed in U.S. Pat. No. 5,909,283, to Eselun. The system described in the '283 patent has a grating scale and readhead including a point source (laser diode in readhead), a Ronchi grating or holographic element, and a photodetector array. As described, the point source results in interference fringes having a spacing equal to that of the scale. The interference fringe light is transmitted through the Ronchi grating or holographic element to the photodetector array. The photodetector array is arranged to derive four channels of quadrature signals from the transmitted fringe light. One drawback of the system described in the '283 patent is that the resulting encoder is of a size that is relatively large or prohibitive for a number of applications, while another drawback is that the system generally only provides for measurements in one dimension. Furthermore, in some modern motion control systems which have optical readheads that use electronic photodetectors, as disclosed in the '283 patent, attenuation of high frequency measurement signals that are detected and transmitted over relatively long wire lengths is becoming a limiting factor.
Another type of relative position optical encoder is disclosed in U.S. Pat. No. 4,733,071, to Tokunaga. The system described in the '071 patent has a code member scale, and an optical sensor head comprising an optical fiber tip light emitter and two optical fiber tip receptors closely arranged along the code member measuring axis. The optical sensor head is rotated (yawed) to adjust phase difference between the two optical fiber tip receptors. However, the accuracy of the resulting encoder is relatively crude, and the system again generally only provides for measurements in one dimension.
Certain of these types of encoders are designed for making 2-dimensional (2D) incremental position measurements. A 2D incremental position encoder using a 2D grating scale and providing high resolution and high accuracy at an arbitrary position in a 2D plane is disclosed in U.S. Pat. No. 5,104,225 to Masreliez. However, the readhead disclosed in the '225 patent is relatively large, complex and expensive. Furthermore, the readhead disclosed in the '225 patent suffers the same general electronic signal attenuation limitations described with reference to the '283 patent.
2D absolute position measuring systems are also known. For example, 2D bar code systems have been adapted for certain low-resolution absolute 2D position measurement applications. However, the “information storage” structures of such 2D bar code systems are generally not well suited to act as a 2D scale for high-resolution position determinations. Furthermore, suitable readheads for such 2D bar code systems are also relatively large, complex and expensive. Furthermore, the electronic signal processing used with such devices severely limits the allowable rates of high speed motion that can be tracked with the devices, in comparison to the speeds required in many modern motion control systems.
A 2D position sensing device that can overcome the foregoing problems and limitations, individually or in combination, is desirable.